The present invention relates to a zoom lens, particularly to a photographic lens for zooming by varying the distance between two lens groups. More particularly, the zoom lens is provided with a photographing aperture movable relative to the rear lens group when the lens group is moved forwards.
Out of the zoom lenses, a two group type zoom lens consisting of a divergent front group and a convergent rear group is disclosed out of U.S. Pat. No. 3,848,969 and others. This kind of zoom lens, being a reverse telephoto type in the arrangement of the refractive power, is desirable provided it has a wide angle range, so that various kinds of improvements have been made in order to obtain a zoom lens for the wide angle range with high image quality.
Despite of the above mentioned superiority, if this type of zoom lens is zoomed toward its wide angle configuration the distance between the front group and the rear group including an aperture becomes so large that the off-axial ray passing through the front group is a large distance from the optical axis so that the diameter of the front lens has to be increased, which is inconvenient.
On the other hand, if the lens is zoomed toward its telephoto configuration, the front and the rear group are close to each other while the rear group including the aperture is moved so far to the object side that the on-axial ray passing through the aperture is a large distance from the optical axis, so that the diameter of the aperture has to be increased, which is inconvenient.
Particularly with reference to the latter problem, in order to decrease the F-number in the telephoto configuration the diameter of the aperture has to be further increased so that the lens barrel containing such an aperture control mechanism becomes unavoidably large, which is not only difficult for easy handling but also not nice to look at.
Under such circumstance, the size of the aperture control mechanism remains unchanged, while the F-number at the telephoto side is choosen larger than that at the wide angle side in order to prevent the increase of the diameter of the lens barrel. However, the F-number changes during the zooming. Thus, the photographic effect intended by the photographer often can not be obtained, which is inconvenient.